Greases possessing good high speed lubricating characteristics are required in a variety of industrial applications, particularly in the lubrication of bearings, such as, for example, electric motor bearings. Products of this type must be capable of lubricating wearing surfaces for extended periods of time at temperatures of as high as 150.degree.C and above, and must, therefore, possess good high temperature bearing life and oxidation stability properties.
In this latter respect, a class of very effective oxidation inhibitors are those of the phenothiazine type, e.g., phenothiazine and various phenothiazine derivatives such as N-substituted and dialkyl substituted phenothiazines, which compounds are normally added as minute crystals. Although these compounds are known to be highly efficacious antioxidants in synthetic lubricants -- see, for example, U.S. Pat. No. 3,803,140 --, they have found only limited application in mineral lubricating oil based greases because of their tendency to form large macroscopic crystals in these products which renders the greases unsuitable for many lubricating applications. These crystals generally form in the finished grease during storage or after use at a high temperature; see, for example, U.S. Pat. No. 3,663,438. In order to prevent phenothiazine-type inhibitor recrystallization, it has been disclosed previously that relatively large amounts of other additives such as polyesters known as recrystallization retarders should also be incorporated in mineral lubricating oil based greases.